1. Technical Field
Aspects of the present invention relate to an image reading device for reading an image from an original.
2. Related Art
A contact image sensor (CIS) is widely used as a reading device in a facsimile device, a copying device and the like. This type of contact image sensor is configured to execute a reading operation in accordance with a start signal and clock pulses from a control unit as disclosed in Japanese Patent Provisional Publications Nos. SHO 54-80024A, SHO 58-6670A, and SHO 60-124176A.
FIG. 10A is a conceptual illustration of a configuration of a contact image sensor unit. As shown in FIG. 10A, the contact image sensor unit includes a light guide 101 provided with a light source 103 such as an LED at its one end, and an image sensor 107. The light guide 101 diffuses light from the light source 103 in a main scanning direction to illuminate an original 105. The image sensor 107 formed to be elongated in the main scanning direction receives light reflected from the original 105.
Since it is difficult to emit light having uniform intensity in the main scanning direction from a light source side edge of the light guide 101 to a predetermined position, output signal levels of the image sensor 107 obtained when a white object is illuminated by the light guide 101 vary depending on a position on the image sensor 107 in the main scanning direction. For example, a maximum difference of 20% to 30% may be caused, on a percentage basis, between white signals which are output signals of the image sensor 107 when a white object is illuminated as shown in FIG. 10B. Therefore, a difference between the level of the white signal and a level of a black signal which is an output signal of the image sensor 107 obtained when the light guide 101 does not emit light becomes smaller in a region of the image sensor 107 having low white signals than that in another region of the image sensor 107 having high white signals. In this case, gray scale reproducibility also decreases in the region of the image sensor 107 having low white signals in comparison with another region of the image sensor 107 having high white signals.